1. Field of the Invention
The present invention relates to an exposure apparatus employed in an optical exposure step of optical exposure technique for forming desired conductive patterns in fabrication of, e.g., printed circuit boards.
2. Description of the Prior Art
When optical exposure technique is applied to fabrication of, e.g., a printed circuit board, a board or panel for forming the printed circuit board must be correctly aligned with a photomask or artwork. Such alignment of the board and the photomask has been generally performed by inserting pins in a plurality of alignment holes provided in each of the board and the photomask.
However, the aforementioned alignment method has the following disadvantages:
(1) Pitch tolerance takes place in working of alignment holes of the board as well as those of the photomask. Also tolerance takes place in inner diameters of the alignment holes, whereby it may be impossible to insert the pins in accurately formed alignment holes of the board and the photomask to be fitted into the same.
Therefore, the pins are provided with play in outer diameter to some extent with respect to the diameters of the alignment holes under the present circumstances, and hence accuracy in alignment of the photomask and the board cannot be improved.
(2) The photomask is generally formed by a thin film of about 0.1 mm in thickness, which film is expanded and contracted by variation in temperature and humidity to reduce the accuracy.
(3) The photomask formed by the thin film is abraded by repeated engagement/disengagement of the pins in/from the alignment holes, whereby it is difficult to retain the accuracy.
With such disadvantage that the accuracy in alignment of the board and the photomask cannot be satisfactorily improved, the conventional alignment method cannot sufficiently cope with the recent printed circuit boards having patterns of higher density and smaller line width.
The conventional alignment method has the following further disadvantages:
(4) Upon insertion of the pins, fragments or chips are caused by abrasion of the board.
(5) In order to obtain multiproduct printed circuit boards, the positions of the pins must be accurately shifted per change in lot, while such operation is relatively difficult and takes time.